Metal oxide and other metal chalcogenide nanoparticles are important functional materials having applications, for example, in optoelectronics and catalysis. The properties of such nanoparticles are influenced by particle size, particle shape, defect concentration and surface groups.
ZnO, in the bulk, is a wide bandgap semiconductor, applied in lasing, field effect transistors, gas sensors and in photovoltaics. The optoelectronic properties of ZnO nanoparticles depend on the particle size, defect concentration and surface species; thus control of these factors is important. Zinc oxide, and other metal oxides (and chalcogenides), are also important in catalysis, for example in the synthesis of methanol from syn-gas. Once again, surface chemistry impacts performance.
Control of surface chemistry is also highly relevant in the preparation of nanoparticle-polymer nanocomposites. These materials are applied in electronics, for example as dielectrics, diodes and the active layer in photovoltaics. They are also applied to protect polymers from radiative decay, as, for example, ZnO absorbs UV radiation, and as luminescent materials. In order to optimise the nanocomposite bulk property enhancements, the dispersion of the nanoparticles in a polymer matrix should be maximised, thereby increasing the particle-polymer interfacial area. The development of suitable fabrication methods that minimise particle aggregation is a key goal. Means to prepare the nanoparticles directly within a polymer/pre-polymer mixture (in situ syntheses), are attractive as they can minimise hard agglomerates often formed during handling of particles synthesised ex situ.
In situ nanocomposite syntheses require preparations for nanoparticles that are compatible with the polymer chemistry, i.e. which operate under mild conditions, are tolerant of chemical functionality, and which generate only by products which are compatible with the polymer system or are easy to eliminate. One common route to ZnO nanoparticles is via the alkaline hydrolysis of zinc halides, (L. Spanhel, J. Sol-Gel Sci. Techn. 2006, 39, 7-24), often accomplished in alcoholic solvents under ambient conditions. Such ‘sol-gel’ syntheses have been used to prepare nanocomposites in situ using certain thermoplastic matrices. However, the method is not generally applicable due to the presence of salt by-products and the lack of compatibility with base-sensitive polymer functionalities, common in reactive thermosets. Zinc oxide nanoparticles may also be prepared by the hydrolysis of organozinc precursors and organometallic hydrolyses are chemically tolerant toward a variety of polymer matrices. The “one-pot”, in situ preparation of bulk ZnO-epoxy resin nanocomposites with improved thermal conductivity, via the hydrolysis of diethylzinc has recently been reported (A. Gonzalez-Campo, et al, Chem. Comm., 2009, 27, 4034-4036).
Effective modification of the nanoparticle surfaces, either in nanocomposites or as materials in their own right, still remains highly challenging (S. Li, et al, Adv. Mater., 2007, 19, 4347-4352). The most common method to control ZnO surface chemistry is via the application of surfactant ligands which are usually applied in great excess. Excess ligands or reactive small molecules are particularly undesirable in nanocomposites where weak interfaces and plasticisation by free surfactant significantly reduce performance.
It has now been determined that a modified hydrolysis process can be used to produce surface-modified nanoparticles without the need for excess surfactant, structure directing agent or ligand, allowing the achievement of homogenous particle size distribution and with control over surface modification. This process is useful for simple, surface-functionalised nanoparticle production, and is particularly applicable for in situ nanocomposite syntheses in which bulk properties can be enhanced by improving nanoparticle dispersion within the composite. The surface-functionalised nanoparticles also have applications in catalysis, for example in the catalysis of methanol production.